1. Field of the Invention
The present invention relates generally to techniques for enabling digital beamforming in radio-frequency (RF) receiver systems and more specifically to methods and apparatus for enabling digital beamforming in an RF receiver system with a multi-element array antenna having short repetitive synchronization sequences in a noise and/or jamming environment.
2. Description of the Related Art
Multi-element array antenna can offer advantages over traditional mechanically steered directional antennas. For example, the use of digital beamforming (DBF) techniques can allow reception from multiple simultaneous streams. DBF algorithms can be used to determine the weight vector, which is used to combine the signals from the antenna elements, resulting in an improved receive signal. Some standard DBF algorithms rely on first and second order statistics, that is, the cross correlation vector and covariance matrix.
As discussed herein, DBF techniques include not only those employed with calibrated phased array antennas, such as phasing up elements to electrically ‘point’ the array, but also those that can be used with multi-element arrays that don't require antenna calibration or a specific antenna geometry and element spacing. One such method is minimum mean squared error (MMSE). This technique uses the cross correlation vector and covariance matrix to calculate a weight vector that minimizes the signal to noise plus interference (SINR) of the combined signal.
In standard approaches, to obtain accurate estimates of the first and second order statistics, it is advantageous to use many samples when calculating the cross correlation vector and covariance matrix. This is particularly relevant when attempting to receive signals in strong noise and/or interference. This can be the case in the presence of a jammer, or in a network allowing co-channel interference, for example, a network with nodes capable of receiving multiple simultaneous signals.
However, it is not always possible to operate on long sequences. The cross correlation vector correlates a sequence on all elements with the ideal known sequence. If the known sequence is short, the correlation output may provide a poor estimate. The length of sequences used for the covariance matrix estimate may be limited by hardware (processing and/or memory).
U.S. Pat. No. 7,289,580, issued to Pladdy, et al., entitled, “Channel Estimator Using One Or More Correlation Reference Vectors to Eliminate Data Related Noise”, discloses a method of estimating the channel impulse response of a channel comprising the following: performing a plurality of correlations, wherein each of the correlations provides a substantially noise-free estimate of the impulse response of a different portion of the channel; and, combining the plurality of substantially noise-free estimates to provide an estimate of the channel impulse response.
U.S. Pat. No. 7,286,800, issued to Maruta, entitled, “Multi-Beam Antenna Reception Device and Multi-Beam Reception Method”, discloses a multibeam antenna reception device capable of improving the reception quality while suppressing an increase in the amount of computation. The multibeam antenna reception device includes a path detection control section for controlling the path detection range at the current time for M receive beam path detection sections based on pairs of receive beam numbers and path delays detected prior to the current time and information on user signal reception quality in the pairs of the receive beam numbers and the path delays output from the M receive beam path detection sections. When path detection is performed with respect to each user in the M receive beam path detection sections, pairs of receive beam numbers and path delays and information on user signal reception quality in the pairs of the receive beam numbers and the path delays are detected according to the path detection range controlled by the path detection control section.
U.S. Pat. No. 6,556,809, issued to Gross, et al., entitled, “Method and Apparatus for Controlling Communication Beams Within a Cellular Communication System”, discloses a beam control subsystem that provides acquisition, synchronization, and traffic beams to communication devices within a footprint of a system node, where each beam comprises a set of beamlets. The subsystem first acquires and synchronizes with each communication device. Acquisition involves selecting and combining sets of beamlets, and determining whether any devices within the sets are attempting to acquire the system. If so, synchronization is performed by varying beamlet weighting coefficients to find, based on modem feedback, a combination of coefficients that yields a maximum signal-to-interference+noise ratio for multiple users within a beam. The communication device is then handed off to a traffic beam. The subsystem continues, based on modem feedback, to adapt beamlet weighting coefficients in order to track the traffic beam in a manner that provides the maximum SINR.
U.S. Pat. No. 7,305,054, issued to Walwar, entitled, “Robust Multiple Chain Receiver”, discloses a method and system for receiving multiple signals at a multiple channel receiver. The receiver is adaptable to receive information signals that are dominated by either noise or interference. The method and system of the invention are implemented with existing multiple channel weighted receivers.